Mode filter devices (hereinafter “mode filters”) are in common use in many modern digital communication systems that utilize optical fibers for data transmission. At the most basic level, a typical digital communication system that utilizes multimode fibers, includes a transmitter for transmitting an optical signal over a multimode fiber connected thereto, and also includes at least one receiver, each for receiving an optical signal of at least one specific desired fiber mode. In order to ensure that the receiver only receives the at least one desired fiber mode, at least one mode filter, each capable of passing at least one desired fiber mode to the receiver, is employed between the multimode fiber and the receiver.
Traditionally, high data bitrate communications were handled with the use of electronic dispersion compensators (EDCs). This approach suffered from a number of drawbacks. First the EDCs are relatively bulky and thus take up valuable space in digital communication system components. Second, such devices require electrical power and fail if the power ceases to be supplied. Third, EDC are relatively expensive. Finally, like all electronic devices such EDCs share the susceptibility to failure from a variety of factors (circuit failure, overheating, electrical surges, physical disturbance, etc.)
To address the above issues, one solution has been proposed in U.S. patent application Ser. No.: 11/524,857, of Deliwala, entitled “High Bitrate Transport Over Multimode Fibers” (hereinafter, the “'857 application”). Specifically, the '857 application disclosed a high bitrate optical signal transport system that utilized at least one substantially in-line “optical fiber-based” mode filter, intended for use between a multimode fiber at one of its ends, and a single mode fiber at its other end, between a transmitter sending an optical signal with many modes over a multimode optical fiber, and a receiver, to ensure that the receiver only receives at least one specific predetermined fiber mode for each mode filter used (See FIGS. 2A, 2B, 3 of '857 application and accompanying descriptions).
As part of the disclosed transport system, the '857 application taught an optical fiber-based mode filter component, shown as a mode filter 2 in FIG. 1 thereof, that, in its primary embodiment comprised, at one end, a multimode fiber taper 8 that is tapered down to a “single mode condition”, and then without interruption expanded, as an adiabatic taper 4, to “match the mode of the single mode fiber at output 6.” The '857 application further stated that the minimum core radius of the taper 4, is “calculated to achieve the single mode condition for given refractive indices of cladding and core”, and further states that, as a result, the output of the taper 4 only transmits the fundamental mode of the multimode fiber. (See FIG. 1 and Paragraphs 17-18 of the '857 application). The '857 application further noted that a single taper 6 could be used without taper 4 with additional supporting components (such as a lens system or on-chip, i.e., electronic, couplers)—however this embodiment is not actually an optical fiber mode filter, because other non-fiber components must be used with all above-described disadvantages thereof.
The '857 application purports to disclose an optical fiber based mode filter 2 (See '857 application FIG. 1) that comprises a two-part adiabatic taper 4 which includes a first taper 4 region at the multimode fiber side I/O 8 (hereinafter, “taper 8”), and second taper 4 region at the single mode fiber side I/O 6 (hereinafter, “taper 6”), respectively.
However, the '857 application mode filter 2, is flawed in several ways. First, the proposed taper-down (taper 8) and then, without interruption taper-up (taper 6) configuration, will not result in sufficient rejection of modes other than the single mode desired within the multimode fiber connected to taper 8. Furthermore, the refractive index contrast between the fiber cladding and the surrounding medium at the taper 8-taper 6 interface, will result in a correspondingly higher number of undesired fiber modes, in addition to the fundamental mode, that would be entering the taper 4, thereby significantly disrupting the operation of the '857 application mode filter 2.
Second, the '857 application clearly states that its adiabatic taper 4 is expanded to “match the mode of the single mode fiber at output 6.” Therefore, even without the above-described inability of the '857 application mode filter 2 to effectively isolate and pass substantially only the fundamental mode, following this teaching, in actuality the taper 6 would be expanded to a much smaller size than the diameter of the MMF or the diameter of SMF.
Referring now to FIG. 1B, a representation of the optical fiber mode filter 2 of '857 application FIG. 1 is shown as a mode filter 100, with tapers 104 and 114 corresponding to '857 application mode filter 2 first taper 4 region at the multimode side I/O 8 (i.e., taper 8), and the second taper 4 region at the single mode side I/O 6 (i.e., taper 6), respectively. The multimode fiber section 102 and the single mode fiber section 16′, correspond to the multimode and single mode fiber segments shown in FIG. 1 of the '857 application, and labeled “MMF”, and “SMF”, respectively. As described in greater detail below, the FIG. 1B herein, accurately shows the actual disparity between the cladding diameters of the '857 application taper 6 and a standard or conventional SMF at the interface therebetween.
While FIG. 1 of the '857 application shows the relative core sizes of the MMF and SMF segments as being relatively similar, in reality, there is typically a much greater disparity in their relative core sizes, while the overall fiber diameter (i.e., cladding size) remains generally similar. It is well known, that each fiber mode has its own corresponding mode field diameter (“MFD”), and for a desired fundamental mode, the MFD is most significantly dependent on core diameter. This means that the fundamental mode of the SMF 16′ substantially corresponds to its core diameter D3′. Therefore, following the teaching of the '857 application, if the taper 6 thereof (taper 114 in FIG. 1B herein) is expanded to match the MFD of the single mode of the SMF at its output, then the diameter D12 of the taper 114 core 108b at its interface with the SMF 16′, is expected to be either equal to, or substantially similar to, the SMF 16′ core 40′ diameter of D3′.
However, as can be readily seen from the FIG. 1B herein, the actual expansion of the diameter of the taper 114 would be rather small, and even at its end (at position Z), the diameter D11 of the taper 114 cladding 110b,would actually be much smaller than the diameter D4′ of the SMF 16′. Such a significant difference between the diameter of the taper at its connecting end, and the optical fiber to which it must be connected, greatly reduces the rigidity and ruggedness of the connection, and thus significantly decreases the mode filter device reliability. Moreover, this arrangement significantly increases the difficulty of centering the mode filter device and the SMF 16′ along the same central longitudinal axis.
By way of example, to further illustrate the above-described flaws in the teachings of the '857 application, a typical commonly available MMF has a cladding diameter of about 125 microns and a core diameter of about 62.5 microns, while a standard SMF has a cladding diameter of approximately about 125 microns, and a core diameter of about 8.3 microns. Referring now to FIG. 1B herein, following the teachings of the '857 application, the taper 112 ('857 application Taper 8) would need to be reduced to a very small core 108a diameter D9 to reach the diameter that supports only at least one specific mode (i.e., the at least one supported mode)—(ignoring for the moment the above-described reason why an uninterrupted taper-taper interface would not actually isolate the at least one specific fiber mode), and then expanded as taper 114 ('857 application Taper 6) until the taper 114 matched the mode of the SMF 16′, which corresponds to expanding the taper 114 until its core 108b diameter D12 at its end, substantially matches the SMF core 40′ diameter D3′.
However, as can be readily seen from FIG. 1B, expanding from the very small diameter core at position Y, such that the core 108b is about 8 microns in diameter D12 at position Z, would result in the diameter D11 of the cladding 11Ob of the taper 114 at position Z being only approx twice the size of the core 108b diameter D12 (i.e., approx 16 microns)—nearly 1/8th  of the diameter of the cladding 42′ of the SMF 16′ at the interface with the taper 114. And, as noted above, this great disparity in relative diameters D11 of the taper 114 and D4′ of the SMF 16′, makes the connection of the mode filter 100 to the SMF 16′ fragile, and thus unreliable.
It should also be noted that the '857 application teaches and describes filtering of only spatial fiber modes. The '857 application mode filter does not have any effect on polarized fiber modes (as it lacks any structure for isolating and/or filtering polarized modes, and further lacks any teaching or suggestion for doing so). As a result, even if the mode filter of the '857 application were capable of performing its recited functions, it would be limited in effectiveness/performance by its reliance solely on spatial fiber mode filtering.
Furthermore, previously known functional mode filters not capable of effective mode conditioning, for signals entering the filter from the single mode fiber side.
It would thus be desirable to provide an optical fiber mode coupling device that provides a high degree of ruggedness when coupled to a conventional optical fiber. It would also be desirable to provide an optical fiber mode coupling device that is capable of substantially isolating at least one desired fiber mode of the optical signal traveling though its central portion by maximizing coupling between the at least one desired fiber mode, and the device's at least one supported fiber mode. It would further be desirable to provide an optical fiber mode coupling device capable of performing the functions of a mode filter for a signal entering its first end, to produce a mode filtered signal at its second end, or as a mode conditioner for a signal entering its opposite second end, to produce a mode conditioned signal at the first end. It would additionally be desirable to provide an optical fiber mode coupling device capable of superior mode filtering and mode conditioning performance by filtering at least one polarized fiber mode in addition to at least one spatial fiber mode.